HEAT vegetable oil in large skillet over medium-high heat; add beef. Cook, stirring occasionally, for 4 to 5 minutes or until no longer pink. Remove from skillet; keep warm.

COOK mushrooms and onion, stirring occasionally, for 2 minutes or until tender. Stir in flour and pepper. Gradually add water and bouillon; stir until bouillon is dissolved. Bring to a boil; reduce heat to low; Cook, stirring constantly, for 2 minutes. Return beef to skillet and heat through; stir in sour cream. Serve over rice or noodles; garnish with green onion.